The present invention relates to a system and method for emitting x-rays, and more specifically, the invention relates to a system and method for administering a desired x-ray dose from an x-ray device.
X-ray emitters such as x-ray catheters typically consist of an anode and cathode assembly mounted in a miniature vacuum tube. During operation, a high DC voltage (15 to 35 kV) is applied to the assembly from a power source. A high electrical field in the anode-to-cathode gaps causes an electron field emission from the cathode surface. The electrons, emitted into the vacuum gap, are accelerated by the electrical field and strike the anode, radiating x-ray energy as they are stopped.
The emission properties of a thermionic cathode, or hot cathode, depend on the temperature of the cathode surface. A hot cathode utilizes an additional electrode providing a low voltage current for heating the cathode surface. The emission properties and the current at the anode are improved by elevating the cathode surface temperature. Furthermore, the anode current and voltage can be controlled and stabilized independently from each other.
A field emission cathode, or cold cathode, is sometimes favored to a hot cathode in medical procedures and other applications. The cold cathode provides a smaller size and lower operating temperature due to the lack of the heating electrode. In a cold cathode, the value of the field emission current is exponential function of the applied voltage. Therefore, the cold cathode cannot provide independent control of the voltage and current.
X-ray catheters are utilized during medical procedures including percutaneous transluminal coronary angioplasty (PTCA) and when irradiation of vessels or body cavities is required. The successful operation of the x-ray catheter requires the ability to administer a precise radiation dose to a target area. The inherent instability of electron emission from cold cathodes provides a technical difficulty in designing power supplies for the catheters. Ideally, the voltage and current should be measured so that the irradiation dose can be calculated in real-time. The calculated dose could then be adjusted to correspond to a desired dose for the given application.
At least two strategies exist for measuring and controlling the irradiation dose emitted from x-ray devices including field emission cathodes. One method monitors and integrates current while voltage is stabilized. The total accumulated dose is calculated as proportional to the electric charge passed through the emitter. A narrow set of stabilized operating voltages (18 to 21 kV), however, is required to maintain the irradiation rate at a nominal value.
Another method utilizes high voltage pulses with stabilized amplitude. The U.S. Pat. No. 6,069,938 issued May 30, 2000 to Chornenky et al. is an example of a method and x-ray device using a pulse high voltage source. In the Chornenky patent, current passing through the x-ray emitter is measured and integrated. Rectangular voltage pulses with stabilized amplitude and known cycle are applied to the emitter. The average electrical current is stabilized by changing the width of the pulses, thus stabilizing the irradiation rate and controlling dose. The technical difficulty of switching high voltage power up and down is not desirable from a manufacturing and cost viewpoint.
The disclosed and other strategies may provide a stable and controlled irradiation dose rate. The current designs, however, have limitations including large unit size and cost, heat generation, and narrow operating voltage range. Therefore, it would be desirable to achieve an x-ray device with a stabilized irradiation rate that overcomes the aforementioned and other disadvantages.
One aspect of the invention provides a system for emitting x-rays comprising: an x-ray emitter, a controller operably connected to the x-ray emitter, a current sensor operably connected to the controller, and a voltage sensor operably connected to the controller. The controller may determine an actual dose rate based on a received current sensor signal and a received voltage sensor signal and may adjust a supplied voltage to allow the actual dose rate to match a predetermined dose rate. The current and voltage sensors measure the current and voltage, respectively, through the x-ray emitter a plurality of times per second. The controller may adjust the actual dose rate based on an irradiation depth by correcting for tissue radiation absorption and an increased radial target area with increasing treatment radius. The controller may further comprise a current integrator operably connected to the current sensor and the controller to integrate instant current values over time to determine an accumulated charge. The actual dose rate may be calculated a plurality of times per second and may be determined according to: D=fxc3x97Ixc3x97(Vxe2x88x92V0)2 wherein D is the actual dose rate at a distance r from the emitter, f is a constant, I is a current through the x-ray emitter, V is a voltage applied across an anode and a cathode, and V0 is a constant.
Another aspect of the invention provides for a method of operating a device for emitting x-rays comprising: applying a voltage from a voltage source to the device, measuring current and voltage within the device, determining an actual dose rate based on the measured current and voltage, comparing a desired dose rate to the actual dose rate, adjusting the applied voltage, and matching the actual dose rate to the desired dose rate. The adjusting of the applied voltage may comprise stabilizing the actual dose rate; an operator may select the desired dose rate.
Yet another aspect of the invention provides for a computer usable medium storing a program for: determining an actual dose rate based on the measured current and voltage, comparing a desired dose rate to the actual dose rate, adjusting the applied voltage, and matching the actual dose rate to the desired dose rate.
The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.